366 WHEELER. (Vor. III. 
the pulsation of the mesodermic folds, long before a special 
heart is formed, a circulation through the body cavity is 
brought about like the circulation in many of the lower worms.”’ 
One of the stages in the formation of the heart is seen in Fig. 
52. The cardioblasts (cé cb) are both more numerous and more 
irregular than in Doryphora. They unite to form a tube, the 
lumen of which is at first oblong in cross-section (Fig. 52 %). 
In the cardiac walls amoeboid cells are occasionally seen (07), 
which loosen themselves from the mesodermic elements and 
pass into the lumen of the tube, probably to form blood cor- 
puscles. 
C. Ectoderm. 
My observations on the organs derived from the ectoderm are 
limited almost exclusively to Doryphora. 
The hypodermis of the embryo secretes two cuticles, the 
second of which covers the larva, while the first is cast off at 
the time of hatching. Shortly before hatching the embryo is 
confined by four loose envelopes, —the chorion, the vitelline 
membrane, the serosa, and the first cuticle. Graber (15) has 
made a similar observation on the embryo Lzma. 
The three broad-based chitinous spines used by the insect in 
rupturing its envelopes, and which are analogous to the frontal 
spine observed in Stvongylosoma by Metschnikow (31) and the 
deciduous claw on the beaks of birds, are secreted by pyramidal 
thickenings of the hypodermis (Fig. 86 sf), the cells of which 
are much lengthened, though forming a single layer. 
In the surface views of the embryo (Fig. 72) it is possible to 
trace the origin of all the ganglia as paired thickenings of the 
outer layer. At first these thickenings differ histologically in 
no particular from the surrounding ectoderm (Fig. 78 77). 
Gradually, however, the cells in the centre of each thickening 
enlarge, while their cytoplasm becomes drawn out into fine 
threads. At the same time all the ganglion cells thus formed 
arrange themselves in such a way as to have their threads inter- 
mingle. Zhzs mass of intertwined threads becomes the Punktsub- 
stanz (Fig. 94 to 104 pct). The outer layer of cells (e) contin- 
uous with the hypodermis (ecd’) stands off somewhat from the 
ganglionic thickenings, leaving a space which is in early stages 
occupied by several large, clear, oval cells (gé/), which divide 
